1. Field of the Invention
This invention relates to a resin bonded permanent magnet and to a curable resin binder therefor.
2. Description of the Prior Art
Permanent magnets containing rare earth elements, such as RCo.sub.5, R.sub.2 Co.sub.17 and RFeB, in which R stands for a rare earth metal element are known. These magnets exhibit a greater amount of magnetic energy products than magnets formed from Alnico alloys or ferrites.
These magnets (hereinafter referred to as "rare earth permanent magnets") are available in two types, i.e., the sintered type and the resin-bonded type. The resin-bonded type magnet has a number of advantages over the sintered-type magnet, including:
(1) A higher degree of dimensional accuracy even when made in a complicated shape; PA1 (2) A higher degree of uniformity in quality and performance; and PA1 (3) A higher yield and a higher degree of machinability.
It, however, has drawbacks, too. It has a relatively low density of magnetic material and its magnetic properties decrease in proportion to the amount of resin used as a binder.
In order to obtain a resin bonded magnet of improved magnetic properties, it is necessary to minimize the amount of resin which is used to bind the magnetic powder containing the rare earth element. It is, however, also necessary a ensure the high adhesive property, heat resistance and mechanical strength.
A thermosetting, thermoplastic, or rubber-like resin has hitherto been used as a binder for making a resin bonded magnet. The thermoplastic and rubber-like resins are used mainly when magnets are made by injection or extrusion molding. The injection or extrusion molding, however, requires the use of a larger amount of resin than other molding does. The molded product has a low packing density of magnetic material and, therefore, low magnetic properties.
It has been considered that the requirements which have hereinabove been stated can be satisfied by a magnet made by compression molding employing a thermosetting resin, particularly an epoxy resin. An epoxy resin is usually employed with a curing agent and exhibits excellent mechanical and adhesive properties. These properties have made epoxy resins useful to be used for making a resin bonded magnet.
Many of the epoxy resins are, however, in the form of a liquid. When a liquid resin is used as a binder for a magnetic powder, it forms secondary particles with the powder, even if it is used only in a small quantity. The formation of secondary particles makes it difficult to satisfactorily supply a mixture of resin and powder into a mold for compression molding by usual methods. The secondary particles are likely to form bridges in the mold, resulting in a lack of uniformity in pressure across the mold, and thereby a lack of uniformity in quality of the molded products and a breakage of the mold.
Under these circumstances, attempts have been made to use a powdered resin binder in order to improve the flowability of a mixture of resin and magnetic powder to obtain a magnet of improved orientation, as disclosed in, for example, Japanese Patent Application laid open under No. 63808/1980. According to this Japanese Patent Application, a powdered resin binder is used to facilitate the movement of particles of a magnetic powder to improve the orientation thereof during compression molding in a magnetic field to produce a magnet of improved magnetic properties. It is true that a mixture of a powdered resin binder and a magnetic powder is easy to supply into a mold, since they are unlikely to form secondary particles. A magnet made by employing a powdered resin binder is, however, lower in mechanical strength than one made by employing a liquid one, since the powdered resin is less uniformly distributed in the magnet. Moreover, the powdered resin which melts when cured forms voids in the molded product which lowers its magnetic properties.